Scoring Low-Energy Electrons

Dear Fluka experts,
I would like to be able to detect electrons at energies below 1keV in
Argon, using the USRTRACk (linear binning, starting at E_min= 0.0) card.
In the manual it is said that in order to obtain good results with low
energy electrons one has to use the MULSOPT card, activating only
single scattering. Now, I am using Flair and somehow it is not
possible to set WHAT(3)=0.0 ("Stretching") of MULSOPT... If I do not
take any of the values of the drag down Fluka crashes.

What am I doing wrong?

Thank you for your help and for the advice I generally received so far!

Johannes